Re: MD5 failing on optical media

On Sun, Aug 04, 2013 at 09:57:47PM +0100, Andrew Smallshaw wrote:
> I've been encountering odd errors whenever I attempt to verify
> files on optical discs. Right now this is on a couple of DVD-RAM
> discs but I've encoutered similar issues on +R and +RW discs in
> the past. Before now I've put it down to real errors and defective
> media but I've caught it in the act now.
>
> If I have data and an MD5 checksum file on an optical disc and
> attempt to verify it in place I get errors even immediately after
> making the checksums. However, if I copy the files onto my hard
> drive first and check those copies against the copied checksum file
> everything is successful:
One possible difference is that cp uses mmap(), while md5 probably
uses read(). It may be that there is a bug in the UDF read code.
You could try copying the file with cat or with dd with various
blocksizes and see if you can get the bad checksums on the file
copied that way.
Thor